Particle image velocimetry (PIV) was applied to compute the velocity field on a weld pool surface... more Particle image velocimetry (PIV) was applied to compute the velocity field on a weld pool surface with an oxide layer. Negative surface tension was observed to drive an inward flow pattern from the boundary to centre of the weld pool; resulting in particles collecting on the centre line of the pool at the surface. Unsteady flow motions were observed by experimental visualization of the particle flow, and the computed velocity field shows strong unsteady interfacial movement during inward flow. These unsteady flow motions is related to clockwise and counter clockwise vortices on the weld pool surface, which in turn affect both mass flow and energy transport. Both the qualitative flow motion and quantitative flow velocity are described in this work, which contributes to explaining the characteristic unsteady fluid flow in the weld pool, the measurements also provides experimental data for validation of numerical fluid flow models of the weld pool.
40th AIAA Plasmadynamics and Lasers Conference, 2009
In this paper, we observed the liquid metal flow motions during laser welding with heat conductio... more In this paper, we observed the liquid metal flow motions during laser welding with heat conduction model. A high speed camera was used to capture the flow motions. The flow behaviors affected by heating source as well as materials themselves; the former can influent the energy density and the latter decided by the material's physical properties. Fluid dynamics behaviors influent heat and mass transfer in the weld pool; the visualization results from high speed camera show that the flow motions can be quite complex when some boundary conditions were changed.
The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional... more The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional results cannot reflect enough information to quantitatively describe the fluid flow in the weld pool; however, there are few direct three-dimensional results available. In this paper, we describe a three-dimensional reconstruction method to measure weld pool surface features based on a single high-speed camera. A stereo adapter was added in front of the high-speed camera lens to obtain two images in the same frame from different view points at the same time. According to machine vision theory, three-dimensional parameters can be reconstructed based on two such images. In this work, three-dimensional velocity fields have been obtained using this method. Based on the calibration technique employed, the associated error is estimated to be less than 11.4%. Quantitative experimental results are useful for understanding the flow pattern, and possibly for controlling the flow of liquid in the weld pool.
Unsteady interfacial phenomena during inward weld pool flow with an active surface oxide
Science and Technology of Welding and Joining, 2009
ABSTRACT Particle image velocimetry (PIV) was applied to estimate the velocity field on a weld po... more ABSTRACT Particle image velocimetry (PIV) was applied to estimate the velocity field on a weld pool surface with an oxide layer. A positive surface tension gradient drives an inward flow pattern from the boundary to centre of the weld pool, resulting in particles collecting on the centre line of the pool at the surface. Unsteady flow motions were observed by experimental visualisation of the particle flow, and the computed velocity field shows strong unsteady interfacial movement during inward flow. These unsteady flow motions are related to clockwise and counterclockwise vortices on the weld pool surface, which in turn affect both mass flow and energy transport. Both the qualitative flow motion and quantitative flow velocity are described in this work, which contributes to explaining the characteristic unsteady fluid flow in the weld pool, the measurements also provide experimental data for validation of numerical fluid flow models of the weld pool.
The linear and mass ablation rates of Ti 2 AlC ceramics under an oxyacetylene flame at a temperat... more The linear and mass ablation rates of Ti 2 AlC ceramics under an oxyacetylene flame at a temperature up to 3000 • C were examined by measuring the dimensions and weight change of the ablated samples. The linear ablation rate was decreased from 0.14 m s −1 for the first 30 s of the ablation to 0.08 m s −1 after 180 s. Ti 2 AlC ceramics gained small amounts of weight upon ablation, which is attributed to the formation of oxidation products on the ablated surface. The ablation surface exhibits a two-layer structure: an oxide outer layer, consisting mainly of ␣-Al 2 O 3 and TiO 2 and some Al 2 TiO 5 , and a porous sub-surface layer containing Ti 2 Al 1−x C and TiC x O y. With increasing ablation time, the content of TiO 2 and Al 2 TiO 5 in the outer layer increased, and more pores developed in the sub-surface layer. The thermal oxidation of Ti 2 AlC under the flame and scouring of the viscous oxidation products by high-speed flow of gas torch are the main ablation mechanisms.
Various physical interfacial phenomena occur during the process of welding and influence the fina... more Various physical interfacial phenomena occur during the process of welding and influence the final properties of welded structures. As the features of such interfaces depend on physics that resolve at different spatial scales, a multiscale and multiphysics numerical modeling approach is necessary. In a collaborative research project Modeling of Interface Evolution in Advanced Welding, a novel strategy of model linking is employed in a multiscale, multiphysics computational framework for fusion welding. We only directly link numerical models that are on neighboring spatial scales instead of trying to link all submodels directly together through all available spatial scales. This strategy ensures that the numerical models assist one another via smooth data transfer, avoiding the huge difficulty raised by forcing models to attempt communication over many spatial scales. Experimental activities contribute to the modeling work by providing valuable input parameters and validation data. Representative examples of the results of modeling, linking and characterization are presented.
The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional... more The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional results cannot reflect enough information to quantitatively describe the fluid flow in the weld pool; however, there are few direct three-dimensional results available. In this paper, we describe a three-dimensional reconstruction method to measure weld pool surface features based on a single high-speed camera. A stereo adapter was added in front of the high-speed camera lens to obtain two images in the same frame from different view points at the same time. According to machine vision theory, three-dimensional parameters can be reconstructed based on two such images. In this work, three-dimensional velocity fields have been obtained using this method. Based on the calibration technique employed, the associated error is estimated to be less than 11.4%. Quantitative experimental results are useful for understanding the flow pattern, and possibly for controlling the flow of liquid in the weld pool.
The effects of translational, figure-eight and double-figure-eight flapping trajectories on the d... more The effects of translational, figure-eight and double-figure-eight flapping trajectories on the dragonfly aerodynamics were numerically studied by solving the Navier-Stokes equations. There is a common characteristic regarding the lift/drag force coefficients that the downstroke flapping provides the lift forces while the upstroke flapping creates the thrust forces for different flapping trajectories. The maximum lift force coefficient exceeds five for the translational trajectory. It is greater than six for the figure-eight and double-figure-eight flapping trajectories, which is sufficiently larger than unity under the steady state flight condition. The ellipse and double-figure-eight flapping trajectories yield the decrease of the lift force, while the figure-eight flapping trajectory yields higher lift force as well as the thrust force than the translational flapping one. During the insect flight, the wing flapping status should be changed instantaneously to satisfy various requirements. Study of the flapping trajectories on the insect aerodynamics is helpful for the design of the Micro-air-vehicles (MAVs).
An optical system was built to visualize unsteady liquid metal flow behaviour during weld pool de... more An optical system was built to visualize unsteady liquid metal flow behaviour during weld pool development. The work piece was heated locally with a defocused, 3kW Nd-YAG laser to form a conduction mode weld pool. A high speed camera, working at a framing speed of 3,000 frames per second was employed to capture the detail of the melt behaviour and the surface flow, dominated by Marangoni forces, during pool formation. The liquid-solid interface was used to record the molten temperature development as a function of time within the area irradiated by the laser. The surface velocity field was evaluated by tracking particle flow. The experimental measurements provide both qualitative information about flow motion and quantitative flow velocity data. Comparing with reported numerical predictions, we find that real flow motions are more complex and unstable than expected during weld pool development.
Geometric relationship between the image and spacial coordinate systems 55 4.1.4 Stereo point rec... more Geometric relationship between the image and spacial coordinate systems 55 4.1.4 Stereo point reconstruction.
The effect of oxygen on transitional Marangoni flow in laser spot welding
Acta Materialia, 2010
ABSTRACT Owing to its significant influence in heat and mass transfer, liquid metal flow during s... more ABSTRACT Owing to its significant influence in heat and mass transfer, liquid metal flow during stationary laser welding has been studied for different concentrations of oxygen in the surrounding environment. Two predominant types of surface flow motion are observed: an inward flow, corresponding to a positive surface tension temperature gradient, and an outward flow, corresponding to a negative gradient. Time-dependent changes in oxygen concentration at the surface are observed to flip the surface tension temperature gradient from negative to positive under appropriate shielding conditions. The oxygen concentration affects not only the flow motion, but also the laser absorption coefficient, which increases with increasing environmental oxygen concentration. It is shown here that the evolution of dissolved oxygen in the weld pool as a function of temperature can have a profound influence on the fluid flow and hence on energy transport.
The steady two-dimensional Navier-Stokes equations with the slip wall boundary conditions were us... more The steady two-dimensional Navier-Stokes equations with the slip wall boundary conditions were used to simulate the supersonic flow in micro convergent-divergent nozzles. It is observed that shock waves can take place inside or outside of the micronozzles under the earth environment. For the over-expanded flows, there is a boundary layer separation point, downstream of which a wave interface separates the viscous boundary layer with back air flow and the inviscid core flow. The oblique shock wave is followed by the bow shock and shock diamond. The viscous boundary layer thickness relative to the whole nozzle width on the exit plane is increased but attains the maximum value around of 0.5 and oscillates against this value with the continuous increasing of the nozzle upstream pressures. The viscous effect either changes the normal shock waves outside of the nozzle for the inviscid flow to the oblique shock waves inside the nozzle, or transfers the expansion jet flow without shock waves for the inviscid flow to the oblique shock waves outside of the nozzle.
Particle image velocimetry (PIV) was applied to compute the velocity field on a weld pool surface... more Particle image velocimetry (PIV) was applied to compute the velocity field on a weld pool surface with an oxide layer. Negative surface tension was observed to drive an inward flow pattern from the boundary to centre of the weld pool; resulting in particles collecting on the centre line of the pool at the surface. Unsteady flow motions were observed by experimental visualization of the particle flow, and the computed velocity field shows strong unsteady interfacial movement during inward flow. These unsteady flow motions is related to clockwise and counter clockwise vortices on the weld pool surface, which in turn affect both mass flow and energy transport. Both the qualitative flow motion and quantitative flow velocity are described in this work, which contributes to explaining the characteristic unsteady fluid flow in the weld pool, the measurements also provides experimental data for validation of numerical fluid flow models of the weld pool.
40th AIAA Plasmadynamics and Lasers Conference, 2009
In this paper, we observed the liquid metal flow motions during laser welding with heat conductio... more In this paper, we observed the liquid metal flow motions during laser welding with heat conduction model. A high speed camera was used to capture the flow motions. The flow behaviors affected by heating source as well as materials themselves; the former can influent the energy density and the latter decided by the material's physical properties. Fluid dynamics behaviors influent heat and mass transfer in the weld pool; the visualization results from high speed camera show that the flow motions can be quite complex when some boundary conditions were changed.
The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional... more The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional results cannot reflect enough information to quantitatively describe the fluid flow in the weld pool; however, there are few direct three-dimensional results available. In this paper, we describe a three-dimensional reconstruction method to measure weld pool surface features based on a single high-speed camera. A stereo adapter was added in front of the high-speed camera lens to obtain two images in the same frame from different view points at the same time. According to machine vision theory, three-dimensional parameters can be reconstructed based on two such images. In this work, three-dimensional velocity fields have been obtained using this method. Based on the calibration technique employed, the associated error is estimated to be less than 11.4%. Quantitative experimental results are useful for understanding the flow pattern, and possibly for controlling the flow of liquid in the weld pool.
Unsteady interfacial phenomena during inward weld pool flow with an active surface oxide
Science and Technology of Welding and Joining, 2009
ABSTRACT Particle image velocimetry (PIV) was applied to estimate the velocity field on a weld po... more ABSTRACT Particle image velocimetry (PIV) was applied to estimate the velocity field on a weld pool surface with an oxide layer. A positive surface tension gradient drives an inward flow pattern from the boundary to centre of the weld pool, resulting in particles collecting on the centre line of the pool at the surface. Unsteady flow motions were observed by experimental visualisation of the particle flow, and the computed velocity field shows strong unsteady interfacial movement during inward flow. These unsteady flow motions are related to clockwise and counterclockwise vortices on the weld pool surface, which in turn affect both mass flow and energy transport. Both the qualitative flow motion and quantitative flow velocity are described in this work, which contributes to explaining the characteristic unsteady fluid flow in the weld pool, the measurements also provide experimental data for validation of numerical fluid flow models of the weld pool.
The linear and mass ablation rates of Ti 2 AlC ceramics under an oxyacetylene flame at a temperat... more The linear and mass ablation rates of Ti 2 AlC ceramics under an oxyacetylene flame at a temperature up to 3000 • C were examined by measuring the dimensions and weight change of the ablated samples. The linear ablation rate was decreased from 0.14 m s −1 for the first 30 s of the ablation to 0.08 m s −1 after 180 s. Ti 2 AlC ceramics gained small amounts of weight upon ablation, which is attributed to the formation of oxidation products on the ablated surface. The ablation surface exhibits a two-layer structure: an oxide outer layer, consisting mainly of ␣-Al 2 O 3 and TiO 2 and some Al 2 TiO 5 , and a porous sub-surface layer containing Ti 2 Al 1−x C and TiC x O y. With increasing ablation time, the content of TiO 2 and Al 2 TiO 5 in the outer layer increased, and more pores developed in the sub-surface layer. The thermal oxidation of Ti 2 AlC under the flame and scouring of the viscous oxidation products by high-speed flow of gas torch are the main ablation mechanisms.
Various physical interfacial phenomena occur during the process of welding and influence the fina... more Various physical interfacial phenomena occur during the process of welding and influence the final properties of welded structures. As the features of such interfaces depend on physics that resolve at different spatial scales, a multiscale and multiphysics numerical modeling approach is necessary. In a collaborative research project Modeling of Interface Evolution in Advanced Welding, a novel strategy of model linking is employed in a multiscale, multiphysics computational framework for fusion welding. We only directly link numerical models that are on neighboring spatial scales instead of trying to link all submodels directly together through all available spatial scales. This strategy ensures that the numerical models assist one another via smooth data transfer, avoiding the huge difficulty raised by forcing models to attempt communication over many spatial scales. Experimental activities contribute to the modeling work by providing valuable input parameters and validation data. Representative examples of the results of modeling, linking and characterization are presented.
The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional... more The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional results cannot reflect enough information to quantitatively describe the fluid flow in the weld pool; however, there are few direct three-dimensional results available. In this paper, we describe a three-dimensional reconstruction method to measure weld pool surface features based on a single high-speed camera. A stereo adapter was added in front of the high-speed camera lens to obtain two images in the same frame from different view points at the same time. According to machine vision theory, three-dimensional parameters can be reconstructed based on two such images. In this work, three-dimensional velocity fields have been obtained using this method. Based on the calibration technique employed, the associated error is estimated to be less than 11.4%. Quantitative experimental results are useful for understanding the flow pattern, and possibly for controlling the flow of liquid in the weld pool.
The effects of translational, figure-eight and double-figure-eight flapping trajectories on the d... more The effects of translational, figure-eight and double-figure-eight flapping trajectories on the dragonfly aerodynamics were numerically studied by solving the Navier-Stokes equations. There is a common characteristic regarding the lift/drag force coefficients that the downstroke flapping provides the lift forces while the upstroke flapping creates the thrust forces for different flapping trajectories. The maximum lift force coefficient exceeds five for the translational trajectory. It is greater than six for the figure-eight and double-figure-eight flapping trajectories, which is sufficiently larger than unity under the steady state flight condition. The ellipse and double-figure-eight flapping trajectories yield the decrease of the lift force, while the figure-eight flapping trajectory yields higher lift force as well as the thrust force than the translational flapping one. During the insect flight, the wing flapping status should be changed instantaneously to satisfy various requirements. Study of the flapping trajectories on the insect aerodynamics is helpful for the design of the Micro-air-vehicles (MAVs).
An optical system was built to visualize unsteady liquid metal flow behaviour during weld pool de... more An optical system was built to visualize unsteady liquid metal flow behaviour during weld pool development. The work piece was heated locally with a defocused, 3kW Nd-YAG laser to form a conduction mode weld pool. A high speed camera, working at a framing speed of 3,000 frames per second was employed to capture the detail of the melt behaviour and the surface flow, dominated by Marangoni forces, during pool formation. The liquid-solid interface was used to record the molten temperature development as a function of time within the area irradiated by the laser. The surface velocity field was evaluated by tracking particle flow. The experimental measurements provide both qualitative information about flow motion and quantitative flow velocity data. Comparing with reported numerical predictions, we find that real flow motions are more complex and unstable than expected during weld pool development.
Geometric relationship between the image and spacial coordinate systems 55 4.1.4 Stereo point rec... more Geometric relationship between the image and spacial coordinate systems 55 4.1.4 Stereo point reconstruction.
The effect of oxygen on transitional Marangoni flow in laser spot welding
Acta Materialia, 2010
ABSTRACT Owing to its significant influence in heat and mass transfer, liquid metal flow during s... more ABSTRACT Owing to its significant influence in heat and mass transfer, liquid metal flow during stationary laser welding has been studied for different concentrations of oxygen in the surrounding environment. Two predominant types of surface flow motion are observed: an inward flow, corresponding to a positive surface tension temperature gradient, and an outward flow, corresponding to a negative gradient. Time-dependent changes in oxygen concentration at the surface are observed to flip the surface tension temperature gradient from negative to positive under appropriate shielding conditions. The oxygen concentration affects not only the flow motion, but also the laser absorption coefficient, which increases with increasing environmental oxygen concentration. It is shown here that the evolution of dissolved oxygen in the weld pool as a function of temperature can have a profound influence on the fluid flow and hence on energy transport.
The steady two-dimensional Navier-Stokes equations with the slip wall boundary conditions were us... more The steady two-dimensional Navier-Stokes equations with the slip wall boundary conditions were used to simulate the supersonic flow in micro convergent-divergent nozzles. It is observed that shock waves can take place inside or outside of the micronozzles under the earth environment. For the over-expanded flows, there is a boundary layer separation point, downstream of which a wave interface separates the viscous boundary layer with back air flow and the inviscid core flow. The oblique shock wave is followed by the bow shock and shock diamond. The viscous boundary layer thickness relative to the whole nozzle width on the exit plane is increased but attains the maximum value around of 0.5 and oscillates against this value with the continuous increasing of the nozzle upstream pressures. The viscous effect either changes the normal shock waves outside of the nozzle for the inviscid flow to the oblique shock waves inside the nozzle, or transfers the expansion jet flow without shock waves for the inviscid flow to the oblique shock waves outside of the nozzle.
Uploads
Papers by chuangxin zhao